1. Field of the Invention
The present invention relates to an optical fiber laser apparatus, more particularly to an up-conversion optical fiber laser apparatus for generating a stable and high-efficient laser beam of a given wavelength.
2. Description of the Related Art
Recently, broad-ranging areas of displays and optical recording devices have witnessed an increasing demand for a laser emitting short wavelength light such as green or blue light. However, disadvantageously, the short wavelength laser involves a difficult manufacture process and a high cost compared to a long-wavelength laser emitting infrared ray or red light.
Therefore, studies have been actively conducted on a manufacturing technique of a laser apparatus for generating short wavelength light from a relatively cheap long-wavelength laser. In a related conventional technology, an up-conversion fiber laser apparatus is disclosed in document “High-power continuous wave up-conversion fiber laser at room temperature” (T.Sandrock and et al., Optics letters Vol. 22, No. 11, Jun. 1, 1997).
The document teaches an optical fiber laser apparatus 10 including an excitation light source 13 having an input light of 830 nm, and an optical fiber 15 having a core 5 doped with rare earth ions of Pr3+ and Yb3+, as shown in FIG. 1(a).
In the optical fiber laser apparatus 10 shown in FIG. 1(b), the excitation light source 13 includes a titan sapphire laser device 11 and a condensing device 12 such as a lens or collimator. An output light of the laser device 11 becomes incident on the core 5 or a clad layer of the optical fiber 15 through the condensing device 12.
In the optical fiber 15, as shown in FIG. 1a, the core 5 doped with Pr3+ and Yb3+ is enveloped by first and second clad layers 4a and 4b. In the optical fiber 15, as shown in FIG. 2, with about 830 nm light incident from the excitation light source 13, electrons of Yb3+ are excited from a ground level of 2F7/2 and then transit to an energy level of 2F5/2. At this time, when the electrons are relaxed to the ground level of 2F7/2, the energy is transferred to Pr3+ near Yb3+ exciting electrons of Pr3+ from a ground level of 3H4 to an energy level of 1G4. In addition, excitation light of 830 nm is directly absorbed into Pr3+, exciting the electrons to an energy level of 3P0. With the electrons relaxed from the excited level, 635 nm red light can be obtained by transition from 3P0 to 3F2 and 520 to 530 nm green light can be generated by transition from 3P0 to 3H5.
Light of 635 nm or 520 to 530 nm wavelengths obtained thereby, as shown in FIG. 1b, resonates between first and second mirrors 16a and 16b of a resonator 17 to oscillate into a laser beam of each wavelength. In general, red light (e.g., 635 nm) exhibits higher gain efficiency than green light (e.g, 520 nm), and thus has a laser oscillation threshold lower than green light. The aforesaid optical fiber laser apparatus oscillates red light of 635 nm due to its preferential oscillation over green light of 520 nm.
Therefore, in order to selectively output a desired low wavelength out of a plurality of light emitting variations, reflectivity properties of a mirror constituting Fabry-perot should be adjusted. That is, to suppress red light emission and oscillate green light in the optical fiber laser apparatus, first and second mirrors are required to have low reflectivity (preferably almost 0%) for 635 nm wavelength and high reflectivity (preferably almost 100%) for 520 to 530 nm wavelength.
However, typically, the mirror cannot ensure a high level of selectivity for the wavelength. Thus, just an adjustment in reflectivity of the mirror hardly enables a desired low wavelength to be selected for oscillation, and still less rarely guarantees a high output. Especially, in case of a small difference in wavelengths as in 635 nm red light and 520 to 530 nm green light, just an adjustment in reflectivity of the mirror hardly allows oscillation of a desired wavelength light.